This application claims the benefit of United Kingdom Application No. 0029286.2 filed Nov. 30, 2000.
This invention relates to a method and apparatus for reducing noise in a tracking error signal, particularly, though not exclusively, for a Digital Versatile Disc (DVD) Read Only Memory (ROM) read system.
In any optical storage mechanism involving rotating media, for example a Compact Disc (CD) or Digital Versatile Disc (DVD), it is usual for a laser to be used to read the data stored on tracks on the media. In order to accurately read the data it is common to provide a mechanism for sensing the position of the laser spot with respect to the tracks. The positional information can then be used to provide a control signal to adjust the laser position. A servo control loop can be established to ensure that the laser remains accurately positioned over the recorded data tracks, regardless of any random or systematic disturbances which are unavoidable in any practical system. When performing a continuous read operation, the laser position needs to be continuously adjusted in order to follow the track as it spirals from in the inner to the outer diameter of the disc.
The signal from the circuitry that senses the laser position is often called the Tracking Error (TE) signal. The function of the TE signal being to sense errors in the laser position and to correct for them. The same signal can also be used to indicate when tracks are being crossed. The signal is also used when the laser is required to seek across many tracks to recover data from another region of the disc.
Sometimes, while carrying out a track jump or seek operation, the TE signal can become quite noisy. In particular, when the laser is positioned midway between two tracks, the TE signal can have a relatively high spike. The introduced noise is undesirable, especially when the TE signal is to be applied to a charge pump, as is common in many ROM tracking error circuit implementations. Furthermore, a noisy TE signal can often cause problems with track counting.
The present invention therefore seeks to provide a method and apparatus for reducing noise on a tracking error signal, which overcomes, or at least reduces the above-mentioned problems of the prior art.
Accordingly, in a first aspect, the invention provides a method of reducing noise in a tracking error signal, comprising the steps of receiving at least two digital input signals from an array of at least two optical detectors, each digital input signal indicative of the amount of laser light incident on the corresponding optical detector reflected from an optical disc, generating a first difference signal when only the first digital input signal is received and a second difference signal when only the second digital input signal is received, detecting when either the first difference signal or the second difference signal is generated, maintaining first or second charging signals while either the first difference signal or the second difference signal is detected, limiting the duration for which the first and second charging signals are maintained to a maximum duration, the maximum duration being programmable according to at least user defined criteria, and generating a tracking error signal from the first and second charging signals.
In one embodiment, the step of detecting when either the first difference signal or the second difference signal is generated comprises the step of XORing the first difference signal and the second difference signal to provide a detection signal.
In one example, the step of limiting the duration for which the first and second charging signals are maintained to a maximum duration comprises the steps of generating a control signal when either the first difference signal or the second difference signal is detected, limiting the generation of the control signal to a duration which is received from a user, and combining the control signal with each of the first and second difference signals to maintain the first or second difference signals only while the control signal is generated.
The maximum duration is preferably further automatically adjusted for a changing data rate in the received at least two digital input signals so as to maintain a predetermined phase difference between the at least two digital input signals. Conveniently, the maximum duration is programmable by the user to change the predetermined phase difference. In one embodiment, the maximum duration is programmable by the user to adjust the predetermined phase difference according to how noisy the at least two digital input signals are so as to reduce noise on the tracking error signal.
In one example, the at least two digital input signals are generated from a 2xc3x972 array of four optical detectors, the at least two digital input signals being generated by summing output signals from diagonal pairs of optical detectors.
According to a second aspect, the invention provides an apparatus for reducing noise in a tracking error signal. The apparatus comprising at least two digital input terminals, a signal difference generator, a programmable timing element and a charge pump for receiving at least two digital input signals from an array of at least two optical detectors. Each digital input signal indicative of the amount of laser light incident on the corresponding optical detector reflected from an optical disc. The signal difference generator having first and second inputs coupled to respective ones of the at least two digital input terminals, a first output for providing a first difference signal when only the first digital input signal is received and a second output for providing a second difference signal when only the second digital input signal is received. The programmable timing element having first and second inputs coupled respectively to the first and second outputs of the signal difference generator, at least a user programmable input, and first and second outputs. The programmable timing element further includes a signal limiting device coupled to the user programmable device for limiting a duration of the first or second difference signals provided at the respective first or second outputs to a user programmable maximum value. The charge pump having first and second inputs coupled respectively to the first and second outputs of the programmable timing element to provide a tracking error signal at an output thereof.
In another embodiment, the programmable timing element comprises a detecting element having first and second inputs coupled respectively to the first and second outputs of the signal difference generator and an output to provide a signal indicating when either the first difference signal or the second difference signal is received.
In one example, the detecting element comprises an XOR gate having a pair of inputs coupled to the outputs of the signal difference generator for receiving the first difference signal and the second difference signal and an output for providing a detection signal when either the first difference signal or the second difference signal is received.
The programmable timing element preferably comprises a control signal generator having a first input coupled to the output of the XOR gate, a second input coupled to the user programmable input, and an output for providing a control signal limited to a maximum duration which is determined by a user, and a combining element comprising a first portion having first and second inputs respectively coupled to the signal difference generator and a third input coupled to the output of the control signal generator and an output for providing the first or second difference signals as controlled by the control signal.
In one example, the programmable timing element comprises a further input for receiving a signal which automatically adjusts the maximum duration of the control signal for a changing data rate in the received at least two digital input signals so as to maintain a predetermined phase difference between the at least two digital input signals. The maximum duration of the control signal is preferably programmable by the user to change the predetermined phase difference. The maximum duration is conveniently programmable by the user to adjust the predetermined phase difference according to how noisy the at least two digital input signals are so as to reduce noise on the tracking error signal.
In one embodiment, the at least two digital input signals are generated from a 2xc3x972 array of four optical detectors, the at least two digital input signals being generated by summing output signals from diagonal pairs of optical detectors.